The manufacture of plastic items by the injection-molding process often leaves excess solidified plastic pieces known as sprues and runners. These, of course, represent plastic that has solidified within the channels through which the molten plastic passes in order to reach the mold cavity for the actually desired item. Although the development of new molding techniques now permits the substantial avoidance of sprues and runners in molding many parts, these techniques do not represent suitable alternatives in all instances, especially for limited production runs. Consequently the production of sprues and runners continues. It is also common to perform part inspections at the molding apparatus and to either manually or automatically reject defective parts and use them for scrap.
These sprues and runners and scrap represent to the manufacturer a source of plastic equal to 8% to 25% of the total material and cannot be wasted. The additional problem arises, moreover, that this plastic, having once undergone the molding process, should only constitute a small portion of the total mix used for further molding. The recycled utilization of the sprues and runners generally requires grinding or other comminuting and subsequent inclusion, in a proper amount, with fresh plastic.
Commonly, an installation for the manufacture of plastic articles includes several molding stations. The raw plastic ingredients generally are blended together in a central blending room from which they can pass to each of the stations in batches, along conveyors or through pneumatic conduits. Collecting the sprues and runners produced at each of the stations, returning them to the central blending room, storing them, subsequently grinding them and attempting to introduce them into the plastic material in proper proportions for molding represents a time-consuming and unpleasant task. Oftentimes, the task simply passes undone. Developing a system that would permit at each station the grinding up of the sprues and runners immediately upon removing them from the desired molded object, and their reintroduction at the station into the stream of plastic for molding and also grinding reject parts represents an appreciable economy both in terms of time and money. However, these economies have yet to be realized.
One approach to providing a system to allow the facile handling of the sprues and runners and reject parts involves grinding at each station and entraining the resultant pellets in a fluid stream for transportation to the apparatus and subsequent combination with fresh plastic. However, this type of system requires the valving of the various fluid streams in order to combine their contents in the correct proportion. It has been found in the course of the work resulting in this invention that a single pneumatic source for two separate supplies of comminuted materials cannot effectively be used with a proportional valve or other continuous proportioning systems. It has been found that the most effective systems involve "on-off" valving.
However, valves designed to control the flow of homogeneous fluids display limitations which preclude their use in systems as proposed above in which the fluids contain suspended solids. Efforts made to overcome these limitations and provide suitable valves for these systems have not met with complete success.
The typical gate valve, which has worked very well for homogeneous fluids, simply does not close on fluids with suspended solids. The pellets prevent the seating of the closing member.
The bladder valve includes a plastic portion forming part of the tube through which the fluid passes. An inflatable bladder includes this plastic portion of the tube. Inflating this bladder produces a constriction of the plastic portion of the tube, eventually closing off the tube entirely.
While the bladder valve possesses the capability to completely close off a fluid stream carrying suspended pellets, it suffers from a drawback of a different nature. The pellets striking the plastic produce both mechanical and thermal destructive forces on the plastic side walls. As a result, the bladder valve experiences a short lifetime when included in systems handling pelletized solids.
The problem becomes further aggravated in the particular systems described above requiring a diverter valve. In these systems, of course, the diverter valve selects from several sources one which will provide the material it contains to a common receptacle. Such a diverter valve also has perhaps a more conventional function of selecting one of several receptacles that the material from a common source will enter. The flapper valve, which has seen frequent use in this type of system, has a rigid flap located on an arm attached to a pivot. The flap can then pivot to close one of two openings from the source into conduits leading to separate containers. Again, however, the rigid flap experiences difficulty in closing upon pellets suspended in the fluid stream and suffers substantial wear and deterioration from bombardment.